feat(server): restructure Pomodoro server into modular components

- move server logic to cmd/server/main.go
- create packages for websocket, pomodoro and broadcast handling
- define models for messages
- remove old GoTomato.go file
- update README

🤖

pkg/models/types.go

@@ -0,0 +1,14 @@
+package models
+
+// BroadcastMessage represents the data sent to the client via WebSocket.
+type BroadcastMessage struct {
+	Mode       string `json:"mode"`        // "Work", "ShortBreak", or "LongBreak"
+	Session    int    `json:"session"`     // Current session number
+	MaxSession int    `json:"max_session"` // Total number of sessions
+	TimeLeft   int    `json:"time_left"`   // Remaining time in seconds
+}
+
+// ClientCommand represents a command from the client (start/stop).
+type ClientCommand struct {
+	Command string `json:"command"`
+}